Underfill of a bumped or raised die utilizing a barrier adjacent to the side wall of flip chip

ABSTRACT

A method and apparatus for attaching a semiconductor device to a substrate. One end of the substrate is elevated to position the substrate and the coupled semiconductor device on an inclined plane. An underfill material is introduced along a wall of the semiconductor device located at the elevated end of the inclined substrate with the underfill material being placed between the substrate and the semiconductor device. An optional but preferred additional step of the invention includes coupling a barrier means to the substrate at a point on the substrate adjacent to a sidewall of the semiconductor device located at the lowest point of the slope created by the inclined substrate. The barrier means prevents the underfill material from spreading beyond the sidewalls of the semiconductor device, particularly in instances where the substrate is inclined at a steep angle.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/619,595,filed Jul. 19, 2000, which is a divisional of application Ser. No.09/304,502, filed May 3, 1999, now U.S. Pat. No. 6,194,243 B1, issuedFeb. 27, 2001, which is a continuation of application Ser. No.08/910,299, filed Aug. 11, 1997, now U.S. Pat. No. 5,973,404, issuedOct. 26, 1999, which is a divisional of application Ser. No. 08/612,125,filed Mar. 7, 1996, now U.S. Pat. No. 5,766,982, issued Jun. 16, 1998.

BACKGROUND OF THE INVENTION

Field of the Invention: The present invention relates to a method andapparatus for underfilling a semiconductor device. More specifically,the present invention relates to a method and apparatus for uniformlyunderfilling a bumped or raised semiconductor chip to be essentiallyvoid free.

State of the Prior Art: Flip-chip and bumped die technology is wellknown in the art. A flip-chip or bumped (raised) die is a semiconductorchip (die) having bumps on the bond pads formed on the active surface orfront side thereof, the bumps being used as electrical and mechanicalconnectors, which is inverted (flipped) and bonded to a substrate bymeans of the bumps. Several materials are typically used to form thebumps on the die, such as conductive polymers, solder, etc. Typically,if the bumps are solder bumps, the solder bumps are reflowed to form asolder joint between the so-called flip-chip and the substrate, thesolder joint forming both electrical and mechanical connections betweenthe flip-chip and substrate. In any event, due to the presence of thebumps on the flip-chip, a gap exists between the substrate and thebottom surface of the flip-chip.

Typically, since the flip-chip and the substrate have differentcoefficients of thermal expansion and operate at different temperaturesand also have different mechanical properties with differing attendantreactions to mechanical loading and stresses, stress develops in thejoints formed by the bumps between the flip-chip and substrate.Therefore, the bumps must be sufficiently robust to withstand suchstressful conditions to maintain the joint between the flip-chip and thesubstrate. To enhance the joint integrity formed by the bumps locatedbetween the flip-chip and the substrate, an underfill material comprisedof a suitable polymer is introduced in the gap between the flip-chip andthe substrate. The underfill also serves to equalize stress placed onthe flip-chip and substrate, helps transfer heat from the flip-chip andhelps protect the bump connections located between the flip-chip and thesubstrate from contaminants such as moisture, chemicals, andcontaminating ions.

In practice, the underfill material is typically dispensed into the gapbetween the flip-chip and the substrate by injecting the underfill alongtwo or more sides of the flip-chip with the underfill material flowing,usually by capillary action, to fill the gap. For example, U.S. Pat. No.5,218,234 to Thompson et al. discloses a semiconductor device assemblywhereby an epoxy underfill is accomplished by applying the epoxy aroundthe perimeter of the flip-chip mounted on the substrate and allowing theepoxy to flow underneath the chip. Alternatively, the underfill can beaccomplished by backfilling the gap between the flip-chip and thesubstrate through a hole in the substrate beneath the chip.

However, the traditional method of underfilling by way of capillaryaction has a serious disadvantage. The small gap formed between theflip-chip and substrate to which it is connected prevents filling thegap in a uniform manner. Such non-uniform underfilling is particularlyprevalent in the areas surrounding the bumps interconnecting theflip-chip to the substrate. When underfilling a flip-chip on a substratesituated in a substantially horizontal plane, the underfill materialwill generally be non-uniform in character and contain bubbles, airpockets, or voids therein. This non-uniform underfill decreases theunderfill material's ability to protect the interconnections between theflip-chip and substrate and environmentally compromises the flip-chipitself thereby leading to a reduction in the reliability of the chip.

A different method of bonding a semiconductor chip to a substrate isdisclosed in U.S. Pat. No. 5,385,869 to Liu et al. whereby the gapbetween the semiconductor chip and substrate is underfilled utilizing asubstrate having a through hole formed therein which is centrallylocated below the semiconductor chip mounted thereon. The through holehas gates or notches formed at each corner thereof which extend beyondthe semiconductor chip, which is mounted thereover. Underfilling the gapbetween the semiconductor chip and the substrate is accomplished byblocking one side of the through hole, applying an encapsulationmaterial on top of and around the chip, and allowing the encapsulationmaterial to flow into the through hole by way of the gates or notches inthe substrate.

As disclosed in U.S. Pat. No. 5,203,076 to Banerji et al., a vacuumchamber is used to underfill the gap between a semiconductor chip and asubstrate. A bead of underfill polymeric material is provided on thesubstrate about the periphery of the chip. Next, the semiconductor chipand substrate are placed within a vacuum chamber with a vacuum beingsubsequently applied to the chip and the substrate to evacuate the gaptherebetween. Air is then slowly allowed to re-enter the vacuum chamberto force the underfill material into the gap between the semiconductorchip and the substrate.

Although the underfill methods disclosed in the Liu and Banerji patentsattempt to address the problem of underfilling in a non-uniform manner,those references present solutions that require specialized substrates,use additional equipment in the underfilling process and increase thecost of production. For example, implementation of the underfillingmethod illustrated in the Liu reference requires the use of aspecialized substrate having a through hole therein. Similarly, theunderfilling method illustrated in the Banerji et al. reference requiresthe use of specialized equipment in the form of a vacuum chamber.

Therefore, it would be advantageous to develop a method for performingunderfill of semiconductor chips which results in underfill materialthat is uniform and substantially free of voids or air. It would be afurther improvement to develop a method for performing uniformunderfilling of the gap between the flip-chip and substrate that is costeffective and utilizes standard substrates.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to an improved method and apparatusfor underfilling the gap between a semiconductor device (flip-chip) andsubstrate. The improved method of attaching a semiconductor device to asubstrate begins with the step of electrically connecting thesemiconductor device to the substrate. Next, one end of the substrate iselevated to a position where the substrate and semiconductor device arelocated on an inclined or tilted plane. Finally, an underfill materialis introduced along the sidewall of the semiconductor device located atthe elevated end of the inclined substrate with the underfill flowinginto and filling the gap formed between the substrate and thesemiconductor device.

The improved method of the present invention may include the step ofusing a suitable dam or barrier located adjacent to the lower edge ofthe inclined substrate, the lowest point of the inclined substrate. Thesuitable dam or barrier prevents the underfill material from spreadingbeyond the sidewalls of the semiconductor device, particularly ininstances where the substrate is inclined at a steep angle with respectto a horizontal plane.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The advantages, as well as other novel and important features of thepresent invention will be better understood when the followingdescription is read along with the accompanying drawings of which:

FIG. 1 is a cross-sectional view of a preferred underfill dispensingstep for a semiconductor device, a bumped flip-chip, and an inclinedsubstrate in accordance with the present invention;

FIG. 2 is a cross-sectional view of another preferred underfilldispensing method, which illustrates the placement of a suitablebarrier, a dam, located adjacent the substrate;

FIG. 3 is a side view illustrating the placement of the semiconductordevice, bumped flip-chip, and substrate of FIG. 1 on top of a supportmember having a vibrator attached thereto;

FIG. 4 is a top view of a semiconductor device, bumped flip-chip, and aninclined substrate illustrating the use of two suitable barriers, dams,to perform the underfill step;

FIG. 5 is a cross-sectional view of another embodiment of the presentinvention, illustrating a backfill method of underfilling the gap formedbetween a semiconductor device, bumped flip-chip, and a substrate;

FIG. 6 is a cross-sectional view of another embodiment of the presentinvention, illustrating a backfill method of underfilling the gap formedbetween a semiconductor device, a bumped flip-chip, and a substratewithout the use of dams; and

FIG. 7 is a cross-sectional view of another embodiment of the presentinvention, illustrating a backfill method of underfilling the gap formedbetween a semiconductor device, a bumped flip-chip, and a substratewherein the substrate is inverted during the underfilling process.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a substrate or chip carrier 10 is shown forconnecting a semiconductor device or flip-chip 12 by conventional directchip bonding techniques. Substrate 10 is typically made of ceramic,silicone, glass, and combinations thereof. Substrate 10 is preferablycomprised of a printed circuit board (PCB) or other carrier, which isused in flip-chip technology, such as an FR4PCB. Substrate 10 has afront end 14, a rear end 16, and a top surface 18, the top surface 18having contact pads thereon.

Flip-chip 12 has a front sidewall 30, a rear sidewall 32, and an activesurface 20. The active surface 20 comprises integrated circuitry and aplurality of contact pads 22. The contact pads 22 have bumps 24 thereon,which provide both electrical and mechanical connection to substrate 10.

An electrical assembly is produced by placing and securing the flip-chip12 on the top surface 18 of substrate 10 having active circuitrythereon. Specifically, the bumps 24 are aligned with the contact pads ofthe active circuitry located on top surface 18 of substrate 10. Theflip-chip 12 is then electrically and mechanically connected to thesubstrate 10 by curing or reflowing the bumps 24, depending upon type ofmaterial comprising the bumps 24. Alternatively, the bumps 24 may beformed on the substrate 10 prior to attachment of the flip-chip. Inother words, either surface may bear the bumps thereon. Although bumps24 are typically formed with solder, it is understood that any othermaterials known in the art (e.g. gold, indium, tin, lead, silver oralloys thereof) that reflow to make electrical interconnects tosubstrate 10 can also be used. Additionally, the bumps 24 may be formedof conductive polymeric and epoxy materials and may include variousmetals being plated thereon.

After reflowing of the bumps 24, a space or gap 26 is formed between theactive surface 20 of flip-chip 12 and the top surface 18 of substrate10. The size of the gap 26 is controlled by the size of the reflowedsolder bumps and typically varies from approximately 3 to about 10 mils.

Next, an underfill material 28 is applied to fill the gap 26 between theflip-chip 12 and the substrate 10. As previously stated, the purpose ofthe underfill material 28 is to environmentally seal the active surface20 of the flip-chip 12 and the bumps 24, help provide an additionalmechanical bond between the flip-chip 12 and the substrate 10 to helpprevent, distribute stress on the flip-chip 12 and bumps 24 and to helptransfer heat from the flip-chip 12. The underfill material is typicallya polymeric material, such as an epoxy or an acrylic resin and maycontain inert filler material therein. The underfill material 28typically has a thermal coefficient of expansion that approximates thatof the flip-chip 12 and/or the substrate 10 to help minimize stressplaced on either the flip-chip 12 or the substrate 10 during theoperation of the flip-chip caused by the heating of the underfillmaterial 28. To promote filling of the gap between the substrate 10 andflip-chip 12, the viscosity of the underfill material 28 is controlledtaking into account the flow characteristics of the underfill material,the material characteristics of the substrate 10, the materialcharacteristics of the flip-chip 12, and the size of the gap 26.

As shown, the underfill process is started by elevating or inclining thefront end 14 of the substrate 10 in order to position the substrate 10on an inclined plane 2 with respect to a horizontal plane 1. The angleof elevation or inclination of the inclined plane 2 and the attendantsubstrate 10 and flip-chip 12 is dependent on the viscosity or the rateof dispensing of the underfill material 28. The viscosity of theunderfill material 28 should be adjusted to allow facile flow of theunderfill material 28 but should be left low enough to readily preventthe flow of the underfill material 28 beyond the perimeter of theflip-chip 12. It should also be understood that the substrate 10 may beinclined by placing the substrate 10 on a support member 44, such as atilted table or conveyor belt, as is shown in FIG. 3 and furtherdescribed below. Alternately, the substrate 10 may be inclined byplacing the substrate 10 below a support member or horizontal plane 1 asdescribed hereinbelow.

Underfilling is accomplished by applying the underfill material 28 underthe front sidewall 30 of flip-chip 12 and allowing it to flow betweenthe flip-chip 12 and the substrate 10 and around the bumps 24. Theunderfill material 28 is applied with an underfill dispenser 34, such asa syringe having a suitable nozzle thereon or any other suitabledispensing means known in the art.

As shown, since the substrate 10 having flip-chip 12 thereon is placedon an incline, in addition to any fluid pressure used to inject theunderfill material and any capillary action force acting on theunderfill material, a gravitational force also acts on the underfillmaterial, causing the underfill material 28 to readily flow from frontsidewall 30 toward rear sidewall 32. Due to the additional action of thegravitational force to that of the injection pressure and capillaryaction, air pockets, bubbles, and voids found within the underfillmaterial 28 are displaced by the denser underfill material 28 as itflows toward the rear sidewall 32 of flip-chip 12. The ability todisplace and the speed of displacement of the voids is dependent on theinclined angle of the substrate 10 having flip-chip 12 thereon, theviscosity of the underfill material 28, the injection rate of theunderfill material 28, and the uniformity of the injection of theunderfill material 28 into the gap between the substrate 10 and theflip-chip 12 to form a substantially uniform flow front of underfillinto and through the gap. If desired, the process of underfilling thegap may be repeated by inclining the substrate 10 in the oppositedirection and subsequently dispensing another amount of underfillmaterial 28 from an opposing side of the flip-chip 12 into the gap toimprove the uniformity of the underfill material 28 filling the gap.

After application of the underfill material 28, the material is curedeither by heat, ultraviolet light, radiation, or other suitable means inorder to form a solid mass.

Referring now to FIG. 2, a second embodiment of an interconnectedflip-chip 12 and substrate 10 is shown. As shown, a dam or barrier 40 isused on the top surface 18 of the substrate 10 to help contain the flowof the underfill from the gap at the rear sidewall 32 of the flip-chip12. Conventional molding equipment and techniques (e.g. pour molding,injection molding, adhesive bonding, etc.) can be used to form the dam40 on the substrate 10. The dam 40 is typically formed from any suitableepoxy resin material compatible with the substrate 10.

The dam 40 extends upwards from and is substantially perpendicular tothe top surface 18 of the substrate 10. As shown, the dam 40 may be seento lay substantially parallel and slightly aft the rear sidewall 32 ofthe flip-chip 12.

The dam 40 limits the expansion or gravitational flow of the underfillmaterial 28 beyond the position of the dam 40. During the underfillprocedure, the underfill material 28 coats and spreads out onto thesurfaces of the flip-chip 12 and substrate 10. The dam 40 prevents thespread of underfill material 28 beyond the rear sidewall 32 of theflip-chip 12 by means of surface tension.

Additionally, use of the dam 40 (as opposed to using no dam) permits useof lower viscosity underfill materials, if so desired, during theunderfill procedure. The underfill material may be easily controlled anda wider range of viscosities may be used by controlling the depth of thedam 40 and by controlling the width between the rear sidewall 32 of theflip-chip 12 and the dam 40. Use of the dam 40 also permits tilting thesubstrate 10 at a greater angle of elevation with respect to thehorizontal plane 1 in order to accelerate the underfill process or topermit the use of higher viscosity underfill materials should such aneed arise. Furthermore, if desired, a dam 40 may be used on all threesides of the flip-chip 12 located on the substrate 10 except the side ofthe flip-chip 12 from which the underfill material 28 is beingdispensed.

Referring to FIG. 3, a side view of a flip-chip 12 and substrate 10,interconnected via bumps 24, of a third embodiment of the invention isshown. The substrate 10 is inclined with respect to a horizontal plane 1by placing the substrate 10 onto a support member 44. Support member 44can be a tilt table, a tilted conveyor belt, or any other means ofsupport suitable for holding the substrate 10 of the present invention.Preferably, support member 44 can be positioned and locked at variousangles and can also be elevated or lowered from front to back as well asside to side.

Attached to the support member 44 is a vibrator 48. The vibrator 48facilitates and hastens the displacement of air pockets and voids by theunderfill material 28 during the previously described underfill process.The action of the vibrator 48 also permits the use of higher viscosityunderfill materials and/or permits underfilling with the support member44 positioned at a gradual slope.

Referring to FIG. 4, a top view of an interconnected solder-bumped 24flip-chip 12 and substrate 10 of a fourth embodiment of the presentinvention is shown similar to that of the second embodiment as shown inFIG. 2. However, this particular embodiment illustrates the use of twodams 40 and 40′, which are oriented transversely with respect to oneanother. The two dams 40 and 40′ lie in substantially parallelorientation with respect to two mutually perpendicular and abuttingsidewalls 50 and 52 of the flip-chip 12.

The method of this embodiment permits underfilling along two sidewalls54 and 56 simultaneously. Dams 40 and 40′ prevent the spread andoverflow of underfill material 28 beyond sidewalls 50 and 52 of theflip-chip 12. The underfill material may be easily controlled and awider range of viscosities may be used by controlling the depth of thedams 40 and 40′, by controlling the width between the sidewalls 50 and52 of the flip-chip 12 and the dams 40 and 40′, and by controlling thedistance between the corners 60 and 60′ of the dams 40 and 40′.

An alternative method comprises tilting the substrate 10 so as toelevate sidewall 54 and applying the underfill material 28 undersidewall 54 via the underfill dispenser 34′. The substrate 10 is thentilted so as to elevate sidewall 56 and the underfill material 28 isdispensed along sidewall 56 via underfill dispenser 34. This alternatingunderfill technique can be repeated until the underfill material 28 isfree of air pockets and voids.

Referring to FIG. 5, a cross-sectional view of an interconnectedsolder-bumped 24 flip-chip 12 and substrate 10 of a fifth embodiment ofthe present invention is shown midway through the underfill process. Inthis particular embodiment, the substrate 10 has a suitable shapedopening 160 situated near the center of the substrate 10 through whichunderfill material 28 can be applied via the underfill dispenser 34.Additionally, dams 40 and 40′ located on each side of the flip-chip 12are molded or suitably attached to top surface 18 of the substrate 10,as described hereinbefore, being positioned to lay slightly beyond firstand second sidewalls, rear sidewall 32, and front sidewall 30,respectively. It should also be understood that other dams 40′ (notshown) are located on the first and second lateral sidewalls of theflip-chip 12 to confine the underfill.

Referring to drawing FIG. 6, a cross-sectional view of an interconnectedsolder-bumped 24 flip-chip 12 and substrate 10 of a sixth embodiment ofthe present invention is shown midway through the underfill process. Inthis particular embodiment, the substrate 10 has a suitable shapedopening 160 situated near the center of the substrate 10 through whichunderfill material 28 can be applied via the underfill dispenser 34. Inthis instance, there is no dam used to confine the underfill material28. Additionally, if desired, the substrate 10 having flip-chip 12located thereon may be tilted in each direction to enhance the flow ofthe underfill material 28 in the gap 26 between the substrate 10 and theflip-chip 12 during the underfilling process.

Referring to drawing FIG. 7, a cross-sectional view of an interconnectedsolder-bumped 24 flip-chip 12 and substrate 10 of a seventh embodimentof the present invention is shown midway through the underfill process.In this particular embodiment, the substrate 10 has a suitable shapedopening 160 situated near the center of the substrate 10 through whichunderfill material 28 can be applied via the underfill dispenser 34.Additionally, dams 40 and 40′ located on each side of the flip-chip 12are molded or suitably attached to top surface 18 of the substrate 10,as described hereinbefore, being positioned to lay slightly beyond firstand second sidewalls, rear sidewall 32, and front sidewall 30,respectively. It should also be understood that other dams 40′ (notshown) are located on the first and second lateral sidewalls of theflip-chip 12 to confine the underfill. In this instance, the substrate10 having flip-chip 12 located thereon is inverted during the underfillprocess so that the underfill material 28 is dispensed through theopening 160 into the gap 26 between the substrate 10 and flip-chip 12.As in the previous embodiments, the substrate 10 is located at an anglewith respect to horizontal plane 1 be although located therebelow andinclined with respect thereto.

In operation, the present method is initiated by elevating or incliningfront end 14 of the substrate 10. As the underfill material 28 is added,in this case by means of an opening 160 through the substrate 10, theunderfill material 28 flows towards the dam 40 and fills the loweredportion of the gap 26 between the flip-chip 12 and the substrate 10. Thefront end 14 of the substrate 10 is then lowered and the rear end 16 ofthe substrate 10 is elevated. The backfill method is then repeated withthe underfill material 28 now flowing towards the dam 40′ to completethe filling of the gap 26 between the flip-chip 12 and the substrate 10.The underfill material 28 is then cured, as previously described.Alternately, the underfill material 28 may be cured after the partialfilling of the gap between the substrate 10 and flip-chip 12, theremainder of the gap filled and subsequently cured.

While the present invention has been described in terms of certainmethods and embodiments, it is not so limited, and those of ordinaryskill in the art will readily recognize and appreciate that manyadditions, deletions and modifications to the embodiments describedherein may be made without departing from the scope of the invention ashereinafter claimed.

What is claimed is:
 1. An assembly of a semiconductor device attached toa substrate, said assembly comprising: an inclined support member, theinclined support member being inclined at an angle in an inclined planewith respect to a horizontal plane; a substrate having a front edge,back edge, first lateral edge, second lateral edge, and a plurality ofelectrical contact pads located thereon, the substrate being located onthe inclined support; a semiconductor device having a plurality of sidesformed by a first side, second side, first lateral edge, second lateraledge, and a plurality of electrical contact pads located thereon; aplurality of electrical connectors interposed between the electricalcontact pads located on the substrate and the electrical contact padslocated on the semiconductor device, thereby electrically andmechanically interconnecting the semiconductor device to the substrate,the plurality of electrical connectors spacing the substrate from thesemiconductor device and causing a gap to be formed therebetween; anunderfill material substantially filling the gap formed between thesubstrate and the semiconductor device; a dam located on the substrateadjacent the second side of the semiconductor device; and a vibratorattached to the inclined support.
 2. The assembly of claim 1, furthercomprising: a second dam located adjacent one of the first lateral edgeand the second lateral edge of the semiconductor device.
 3. The assemblyof claim 1, further comprising: an aperture extending through thesubstrate at a location adjacent the semiconductor device and extendingbetween at least two of the electrical connectors of the plurality ofelectrical connectors extending between the substrate and thesemiconductor device.
 4. The assembly of claim 1, wherein the underfillmaterial flows into and through the gap by the substrate being supportedon the inclined support with the underfill material thereby flowingdownwardly from the first side of the semiconductor device in adirection of the second side.
 5. An assembly of a semiconductor deviceattached to a substrate, said assembly comprising: a substrate having afront edge, back edge, first lateral edge, second lateral edge, and aplurality of electrical contact pads located thereon, the substratebeing located on an inclined support on an inclined plane with respectto a horizontal plane; a semiconductor device having a plurality ofsides formed by a first side, second side, first lateral edge, secondlateral edge, and a plurality of electrical contact pads locatedthereon; a plurality of electrical connectors interposed between theelectrical contact pads located on the substrate and the electricalcontact pads located on the semiconductor device, thereby electricallyand mechanically interconnecting the semiconductor device to thesubstrate, the plurality of electrical connectors thereby spacing thesubstrate from the semiconductor device and causing a gap to be formedtherebetween; an underfill material substantially filling the gap formedbetween the substrate and the semiconductor device, the underfillmaterial substantially uniformly filling the gap by including the forceof gravity acting thereon during the filling of the gap; a dam locatedon the substrate adjacent the second side of the semiconductor device;and a vibrator attached to the inclined support for vibrating thesubstrate.
 6. The assembly of claim 5, further comprising: a second damlocated adjacent one of the first lateral edge and second lateral edgeof the semiconductor device.
 7. The assembly of claim 5, furthercomprising: an aperture extending through the substrate at a locationadjacent the semiconductor device and extending between at least two ofthe electrical connectors of the plurality of electrical connectorsextending between the substrate and the semiconductor device.
 8. Theassembly of claim 5, wherein the underfill material flows into andthrough the gap by the substrate being supported on an inclined supportwith the underfill thereby flowing downwardly from the first side of thesemiconductor device in a direction of the second side.